TABLE OF CONTENTS The cost of a solution to the structural-coupling problem, in terms of the additional phase lag introduced into the FCS, is not insignificant when compared with the lag from other sources such as primary actuators, as indicated by the example of Figure 7.5. As expensive development efforts are applied to reduce overall system lags in order to realise the performance potential of the FCS, a commensurate effort must be made to minimise the cost of the SC solution. The approach to structural coupling at British Aerospace is therefore under continual review; a number of the directions where work is required or underway are outlined below.
For consistency with the factors motivating the design and clearance methodology applied to EAP, structural coupling has been treated as a flight-safety issue. As already described, however, it is unlikely that structural failure could result directly from a structural-coupling encounter, because of the limited energy which the FCS can input, and because of system's constraining nonlinearities. For a flexible aircraft, an unstable mode will manifest itself as a limit-cycle oscillation in these circumstances, which may be very undesirable, but is not necessarily critical to flight safety. The real concerns are with the coupling of the flutter modes, structural and FCS hardware fatigue and effects on actuator performance, which may themselves have safety implications.
To fully understand these concerns, a method must be devised for predicting the amplitude and frequency of a limit-cycle oscillation...
